Re: [PATCH v6 08/15] net: add helper executing custom callback from napi

[Pavel Begunkov <asml.silence@xxxxxxxxx>]

On 10/21/24 15:25, Paolo Abeni wrote:
> Hi,
>
> On 10/16/24 20:52, David Wei wrote:
> > @@ -6503,6 +6511,41 @@ void napi_busy_loop(unsigned int napi_id,
> >   }
> >   EXPORT_SYMBOL(napi_busy_loop);
> >   
> > +void napi_execute(unsigned napi_id,
> > +		  void (*cb)(void *), void *cb_arg)
> > +{
> > +	struct napi_struct *napi;
> > +	void *have_poll_lock = NULL;
>
> Minor nit: please respect the reverse x-mas tree order.
>
> > +
> > +	guard(rcu)();
>
> Since this will land into net core code, please use the explicit RCU
> read lock/unlock:
>
> https://elixir.bootlin.com/linux/v6.12-rc3/source/Documentation/process/maintainer-netdev.rst#L387

I missed the doc update, will change it, thanks


> > +	napi = napi_by_id(napi_id);
> > +	if (!napi)
> > +		return;
> > +
> > +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> > +		preempt_disable();
> > +
> > +	for (;;) {
> > +		local_bh_disable();
> > +
> > +		if (napi_state_start_busy_polling(napi, 0)) {
> > +			have_poll_lock = netpoll_poll_lock(napi);
> > +			cb(cb_arg);
> > +			local_bh_enable();
> > +			busy_poll_stop(napi, have_poll_lock, 0, 1);
> > +			break;
> > +		}
> > +
> > +		local_bh_enable();
> > +		if (unlikely(need_resched()))
> > +			break;
> > +		cpu_relax();
>
> Don't you need a 'loop_end' condition here?

As you mentioned in 14/15, it can indeed spin for long and is bound only
by need_resched(). Do you think it's reasonable to wait for it without a
time limit with NAPI_STATE_PREFER_BUSY_POLL? softirq should yield napi
after it exhausts the budget, it should limit it well enough, what do
you think?

The only ugly part is that I don't want it to mess with the
NAPI_F_PREFER_BUSY_POLL in busy_poll_stop()

busy_poll_stop() {
	...
	clear_bit(NAPI_STATE_IN_BUSY_POLL, &napi->state);
	if (flags & NAPI_F_PREFER_BUSY_POLL) {
		napi->defer_hard_irqs_count = READ_ONCE(napi->dev->napi_defer_hard_irqs);
		timeout = READ_ONCE(napi->dev->gro_flush_timeout);
		if (napi->defer_hard_irqs_count && timeout) {
			hrtimer_start(&napi->timer, ns_to_ktime(timeout), HRTIMER_MODE_REL_PINNED);
			skip_schedule = true;
		}
	}
}

Is it fine to set PREFER_BUSY_POLL but do the stop call without? E.g.

set_bit(NAPI_STATE_PREFER_BUSY_POLL, &napi->state);
...
busy_poll_stop(napi, flags=0);


--
Pavel Begunkov